Drilling very deep boreholes or enhancing existing wells in hard rock far below the earth's surface, e.g. 10,000 feet deep or more, is inherently incompatible with traditional mechanical or contact drilling or rock removal technologies. Low rates of penetration, extreme bit and drill string wear, and excessive time spent “tripping” to replace damaged or worn bits and drill string make conventional rotary and coiled tubing drilling economically non-viable for many deep, hard rock applications.
Several non-contact techniques have been developed for hard rock drilling but may be effective only in shallow and/or air filled boreholes. Most notably, air or flame jet spallation drilling uses a hot gas or flame directed against a rock surface to cause spalling and removal of the rock. This technique, however, is only feasible in shallow, air-filled boreholes. To drill deeper, a borehole must be filled with water or “mud” to provide mechanical stability. In this environment, flames are not viable in part because of the difficulty in generating or maintaining the required flame under the high pressure water column. For example, the high pressures at the bottom of deep, fluid-filled boreholes make behavior of the flames extremely unstable and difficult to maintain. Further, initiating combustion under these conditions is extremely challenging and typically requires an energy source to be provided at the bottom of the borehole. However, using an energy source such as a spark or glow plug would require, e.g., a power cable to be run from the surface, which is not feasible in deep applications. Other energy sources such as flame holders are inherently unstable, especially at such depths.
Further, most combustion reactions produce very high temperature flames, typically 1800-3000° C. or more. Such temperatures can destroy drilling components and require careful addition of cooling water to maintain a temperature that can be withstood by downhole tools. In addition, such high temperatures can melt rock (e.g., into an amorphous glass) so that the rock is then unspallable. Even a momentary interruption in cooling water can transform rock so that it can no longer be spalled and/or destroy downhole components, even if a cooler temperature is recovered. Small changes in the stand-off distance, or distance from the combustion to the rock surface, can result in dramatic changes in the nature of the high temperature flame impingement, which may result in a temperature too low for spallation, or temperatures high enough to soften or melt the rock. Such tight tolerances for stand-off distances are difficult to control at the bottom of a deep borehole.
Further, flame-based combustion systems require multiple conduits for fuel, oxidant and cooling or circulating water. Other approaches to spallation drilling such as the use of electrical heating require sufficient power down hole. In deep drilling operations, multiple conduits or supply of sufficient power through cables from the surface or through transformation of energy by hydraulic flow may not be feasible, or may be simply impossible.
Combustion systems that require the use of gaseous oxidants, such as air or oxygen, are also unsuitable for deep fluid filled borehole conditions, in part because the pressures required to pump these gases against a hydrostatic column of a fluid filled borehole are sometimes impossible to achieve, and even if possible, have associated safety risks.
There is a need for a method that fulfills the promise of thermal spallation drilling in high pressure, water filled boreholes. If the challenge of drilling deep boreholes in hard rock is not solved, EGS may not become the much needed clean alternative to meeting our current and future global energy needs.